Poland doesn't actually exist as a nation in ww1. It was created after the end of the war by the Peace treaties. The ruler of where the Poles lived then was the Czar of Russia, it was part of the Russian Empire.
The ruler of Russia during World War 1 was Tsar Nicholas II. He was Russia's last Emperor and, along with the rest of the royal family and even their servants, was executed by communist revolutionaries on July 17, 1918.